This invention relates in general to electric arc heaters and in particular to non-transferred electric arc heaters capable of high power operation for extended periods of time.
Electric arc heaters designed for industrial applications are used to heat a wide range of gas compositions to high temperatures. The high temperature gases can be used for heating a furnace or for chemical or metallurgical processes. Typically, these arc heaters are designed for flange mounting to an opening on the furnace or chemical reactor with the arc-heated gas discharge end terminating at the flange attachment or protruding through the wall of the furnace or reactor. Examples of this type of arc heater may be found in U.S. Pat. No. 3,705,975, entitled "Self-Stabilizing Arc Heater Apparatus", issued Dec. 12, 1972 and U.S. Pat. No. 4,214,736, entitled "Arc Heater Melting System" issued July 29, 1980, both patents assigned to the assignee of the present invention. The arc heaters described in these patents include features such as water-cooled axially spaced electrodes having small electrode gaps for simple arc starting and stabilization and water-cooled field coils for rotating the arc over the surfaces of electrode to reduce water and erosion caused by the arc. Power levels of up to 3 megawatts have been obtained in commercial applications of this type of arc heater. However, for many industrial applications where conversion to electrical heating is economically viable, the total heating requirement may be in the range of 10 to 40 megawatts or higher. An electric arc heater capable of higher power operation would minimize the total number of units and associated equipment required for these higher power applications; thus, simplifying the overall installation.
By simultaneously increasing the gas flow rate and lengthening the downstream electrode, it is believed that power levels of these existing designs of arc heaters could be increased to reach these higher power levels. However, with this approach, the downstream electrode would be heavier, more cumbersome to replace and more expensive to manufacture. Further, the length of the downstream electrode required for these higher power levels would be longer than the average arc length due to the tendency of the arc to continuously restrike at various positions along the length of the downstream electrode. This variation in arc length, which can be significant where the length of the electrode is a significant proportion of the maximum arc length achievable in the arc heater, causes power fluctuations that decrease operating efficiency. In addition, because of the large heat transfer surface presented by the downstream electrode, the efficiency of the electric arc heater is further reduced. Therefore, it would be advantageous to have an electric arc heater which can operate at these high power levels at a reasonable level of efficiency (typically 80% or greater). The design should also inhibit restriking of the arc to maximize arc length and power within the arc heater.
One solution to maximize arc length and inhibit arc restrike on the electrode has been to incorporate one or more interelectrode segments between the two electrodes of the arc heaters. Examples of this construction can be found in U.S. Pat. No. 3,953,705, entitled "Controlled Arc Gas Heater" issued Apr. 27, 1976 and in British Patent Specification No. 1,360,659, published July 17, 1974, entitled "Heating Device". Both designs utilize one or more interelectrode segments between the two electrodes in order to increase arc length. The segments are electrically insulated from the electrodes in order to minimize the occurrence of arc restrike.
For maximum heat transfer from the arc to the gas, and therefore for maximum arc voltage, the passageway formed by the interelectrode segments is reduced in diameter. This constricts gas flow, increases turbulence; thus, maximizing heat transfer. With these designs, because the diameter of the constriction is substantially less than the diameters of the electrodes, the pressure of the gas therein is kept at a high value. This in turn demands a greater potential difference between the two electrodes of the arc heater in order to maintain the arc, because the voltage gradient in the arc heater is proportional to the square root of pressure, the total power input to the gas is increased by maintaining a high arc pressure. The increased power input increases the net energy transferred to the gas that is being heated. Although high power operation is achieved, high gas pressures, typically on the order of 1500 psig, are required. These high pressures necessitate more elaborate gas supply systems including costly high pressure compressors. Thus, it would be advantageous to have a high power arc heater capable of operating at lower gas pressures. Further, because of the high power level of these devices, electrode life is relatively short and is measured in terms of a few hours. This short electrode life is unacceptable for industrial applications. Therefore, it would be advantageous to have a high power arc heater having electrode life measured in terms of hundreds of hours instead of just hours. Because the passageway through the interelectrode segments is substantially smaller than the diameters of the electrodes that are used, initiation of the arc can be difficult. A high power arc heater in which are initiation is facilitated by the design of the interelectrode segments would also be advantageous.
One object of the present invention is to provide a high power electric arc heater having electrode life which is acceptable in an industrial environment. Another object of the invention is to provide an arc heater in which arc initiation is facilitated, and one in which arc strikeover to the interelectrode segments is minimized. A further object of the invention is to provide a high power arc heater capable of operating on gas pressures substantially less than 1500 psig.